The present invention relates to an improvement on a binder for hot coating foundry sand containing a solid resol phenolic resin as principal ingredient thereof.
The resin coated sand obtained by the hot coating method has conventionally been prepared by mixing a novolak phenolic resin (hereinafter referred to as novolak resin) with heated sand particles, adding under agitation an aqueous solution of hexamethylene tetramine (hereinafter referred to as hexamine) as a hardener in an amount of up to about 15 percent by weight, usually about 10 to 15 percent by weight with respect to the resin, and further adding calcium stearate. The resin coated sand thus prepared with a novolak resin and hexamine is advantageous in having a fast hardening speed and a high flowability, but the hexamine employed in this process is thermally decomposed during the mold formation or when casting metal to form nitrogen compounds such as ammonia or formaldehyde, thus leading to a pollution problem, and the nitrogen compounds resulting from thermal decomposition of hexamine give rise, when applied to iron or steel casting, to defects such as pinholes or blow holes in the cast products. In order to avoid such drawbacks there has been proposed the use, in the hot coating method, of various binders with very low content of nitrogen or completely free from nitrogen. As one of such binders there was an attempt, already 20 years ago, of the use of a solid resol phenolic resin (hereinafter referred to as solid resol resin) alone as the binder for the hot coating method. Such material is however deficient in comparison with the system of novalak resin and hexamine in slower resin hardening speed and still in a partial hardening phenomenon of the resin occurs while the sand particles were not sufficiently coated during the mixing step, thus leading to an insufficient resin flow (insufficient hot flowability). As a result the formed product shows a strength significantly lower than that obtainable in the system of novolak resin and hexamine and has not, therefore, been employed in practical use. Said material has however attracted attention again as a solution for the gas defects such as pinholes or blow holes and the polluting disagreeable odor at mold formation or metal casting, resulting from the use of hexamine as explained in the foregoing.
It has already been disclosed in copending patent application Ser. No. 865,558, filed Dec. 29, 1977 now U.S. Pat. No. 4,196,114, the disclosure of which is incorporated herein by reference, that the above-mentioned drawbacks can be avoided by adding a lubricant to the solid resol resin. The effect of said addition is quite significant, but the present inventors have found that said effect can be further improved by forming the solid resol resin into a suitable shape and have thus achieved the present invention.